Conventionally, an ejector is known, in which a fluid is drawn from a fluid suction port by a suction action of a jet fluid jetted from a nozzle for decompressing and expanding the fluid to be jetted. In this kind of ejector, the velocity energy of the mixture between the jet fluid and the suction fluid drawn from the fluid suction port is converted to the pressure energy in a pressure increasing portion (e.g., diffuser portion), so that the pressure of the fluid flowing out of the ejector is increased more than the pressure of the suction fluid.
In order to sufficiently increase the pressure of the fluid in the pressure increasing portion of the ejector, it is prefer to increase the flow velocity of the jet fluid, thereby to effectively increase the flow velocity of the mixed fluid in the ejector. Thus, in a conventional ejector, there is provided with a technical means for improving an energy conversion efficiency (hereinafter, referred to as “nozzle efficiency ηnoz”) when the pressure energy of a fluid is converted to a velocity energy of the fluid in a nozzle.
For example, Patent Document 1 (JP 11-37577A) describes regarding an ejector in which first and second throat portions (throttle portions) for reducing fluid passage sectional areas are provided in a fluid passage of a nozzle.
In the ejector of Patent Document 1, an expanding angle of a fluid passage downstream of the second throat portion is reduced near the fluid jet port, so as to restrict a generation of gas and liquid separation and a generation of a scroll flow in the fluid passage downstream of the second throat portion, thereby improving the nozzle efficiency ηnoz.
The nozzle efficiency ηnoz is specifically defined by the following formula F1.ηnoz=(Vnoz2/2)/Δinoz  (F1)
Here, the Vnoz is the velocity of the jet fluid, and Δinoz is a decrease amount of a special enthalpy when a fluid of per weight unit is decompressed and expanded in iso-entropy in the nozzle. That is, the Δinoz is a difference of the special enthalpy between the enthalpy of the fluid at the inlet of the nozzle and the enthalpy of the fluid at the outlet of the nozzle.
In the ejector of the Patent Document 1, it is the pre-condition in which the fluid flowing into the first throat of the nozzle is in a liquid state. However, in the ejector of the Patent Document 1, if a gas-liquid two-phase fluid flows into the first throat of the nozzle, it is difficult to improve the nozzle efficiency ηnoz.
The reasons will be described with reference to FIGS. 13A and 13B. FIG. 13A is a Mollier diagram for explaining a decompression stage of a liquid fluid decompression in the nozzle of the ejector, and FIG. 13B is a Mollier diagram for explaining a decompression stage of a gas-liquid fluid decompression in the nozzle of the ejector. In addition, in FIGS. 13A and 13B, the dashed lines show the isoentropic curved line.
The ηinoz in the above formula F1 is a value determined by the physicality of the fluid. Thus, in order to improve the nozzle efficiency ηnoz, it is necessary to increase the Vnoz by decreasing the loss while the fluid is decompressed in the nozzle. Therefore, it is desirable for the fluid to be decompressed in the nozzle along the isoentropic curved line.
Furthermore, as shown in FIGS. 13A, 13B, the isoentropic curved line is an approximately S-shaped curved line, in which a decrease degree of the enthalpy becomes gradually smaller as the pressure decreases when the liquid fluid is decompressed to become in a gas-liquid two-phase state, and the decrease degree of the enthalpy becomes gradually larger as the pressure decreases when the gas-liquid two-phase fluid having a relatively small pressure is further decompressed.
In the ejector of Patent Document 1, when the liquid fluid flows into the nozzle (i.e., the first throat), the fluid is decompressed approximately along the isoentropic curved line in the entire decompression as shown in FIG. 13A even when the expanding angle of the fluid passage downstream of the second throat is made small near the fluid jet port.
In contrast, in a case where gas-liquid two-phase fluid having a relatively low pressure flows into the nozzle, it is difficult to perform a decompression stage approximately along the isoentropic curved line, as shown in FIG. 13B. As a result, when the fluid flowing into the first throat portion of the nozzle of Patent Document 1 is in the gas-liquid two-phase state, it is difficult to improve the nozzle efficiency ηnoz.